1. Field of the Invention
The present invention relates to a process for dewatering aqueous sludges of organic nature by adding thereto flocculants.
2. Description of the Prior Art
There had been proposed a process for dewatering aqueous sludges of organic nature, such as sludges from sewage treatment, biological treatment of night soil and so on, in which a cationic organic flocculant is first added to the sludge with agitation and thereto is then added an anionic organic flocculant with agitation to effect flocculation, so as thereafter to subject the formed flocs to dewatering operation. If ordinary synthetic organic flocculants are employed in this process as the cationic and anionic flocculants, the tolerable ranges for the dosages of such flocculants and for the dosage ratio between these flocculants will be considerably restricted, so that accommodation in the practical operation to the widely fluctuating actual conditions, such as the concentration, nature and properties of the sludge etc., is difficult. Moreover, this prior process exhibited a further shortcoming in that, though the flocs formed are large and stout, they are sticky and the moisture content of the dewatered sludge cake is higher as compared with that of the cake obtained by the sole addition of a cationic or anionic flocculant, resulting in a decrease of the filtration velocity.
In order to remedy these difficulties, an improved process was proposed in which for either one of the cationic and anionic flocculants a natural organic polymer derivative is used. Though this process was excellent insofar as it brought forth improvements of the difficulties mentioned above, there was some fear for the stable supply of the raw material for the natural organic polymer derivative. In particular, while it is attainable to lower the moisture content of the dewatered sludge cake by a drastic degree by employing a natural organic polymer derivative, especially chitosan, for the cationic organic flocculant, there is no satisfactory assurance of the stable supply of the raw material therefor, since chitosan is obtained from crab exoskeleton.
Attempts have been made to replace such natural occurring organic flocculant by a synthetic polymer offering a comparable dewatering performance, and there was proposed a process in which a polymer having a constituent unit of imidazoline was employed. However, it has been expected that a synthetic polymer realizing a more effective dewatering performance will be found.